The present invention relates, in general, to the field of surface acoustic wave devices, and in particular, to a system for providing precision filters and oscillators that uses a curved transceiving element.
Surface acoustic wave (SAW) devices are used extensively in modern electronic systems, especially in those involving communications or signal processing applications. A SAW device is formed starting with a piezoelectric substrate, onto the surface of which electrodes (also referred to as antennas) or other transceiving elements are patterned using known photolithographic processes. Typically, a transmitting set and a receiving set of multiple, inter-digited electrodes are formed into a lattice configuration.
The inter-digited electrodes now disposed upon the piezoelectric substrate operate to convert a voltage into a surface acoustic wave, or a surface acoustic wave into voltage. Specifically, as voltage is applied to the transmitting electrodes, an acoustic wave is formed in the substrate, due to the piezoelectric effect. The acoustic wave propagates in the substrate at a given velocity that differs depending upon the direction of propagation. Many devices operate in the surface mode, in which the relevant velocity is the surface acoustic velocity. When the acoustic wave reaches the receiving electrodes, the acoustic energy is reconverted to electric energy.
Designers may thus employ SAW devices to provide filter and oscillator functions in a signal processing or communication application. In such applications, a technical measure of SAW efficacy may be referred to as Quality factor (Q). Quality factor may be calculated by the formula Q=xcfx89/xcex94xcfx89; where xcfx89 represents the frequency of operation and xcex94xcfx89 represents the variance of the frequency. Variance of the frequency may be affected by a number of factors including transmission losses, phase jitter, and other distortions which may be intentionally and unintentionally introduced.
For a given frequency, the quality factor Q may be increased providing more precise filters and oscillators. This may be accomplished by increasing frequency or by decreasing the variance (or a combination of the two). Variance increases with losses in transmission of the acoustic wave, including diffractive losses. It is critical, therefore, to curtail diffractive losses in high precision SAW applications.
It has been found that in conventional SAW devices employing linear electrodes, the diffractive losses are unnecessarily high which causes a high variance and a low Q value. This phenomenon is illustrated in FIG. 1 wherein a conventional SAW device 100 uses linear electrodes. Transmitting electrode 102 propagates waves towards receiving electrode 104, as shown by wave fronts 106. As waves 106 are propagated from electrode 102, they are diffractively altered in size and shape over the course of transmission. As they arrive at electrode 104, waves 106 exceed the receiving area of electrode 104, resulting in portions 108 of the wave being lost.
A further problem with straight electrodes pertains to phase delay errors caused by a variance in arrival time of the acoustic signal across the receiving electrode. In the standard SAW device configuration of parallel, straight transmitting and receiving electrodes, the leading phase front of an acoustic signal arrives at the center of the receiving electrode before arriving at the tips of the receiving electrodes. This spread, or dispersion, in arrival time lowers performance of the device by increasing xcex94xcfx89 and lowering Q.
Some conventional systems have attempted to address this by significantly increasing the size of receiving electrodes. This approach, however, has unacceptable impacts on system efficiency and costs. Other conventional systems have used shaping and positioning of electrodes in an attempt to reflect acoustic waves. Mere redirection, however, fails to address diffractive losses, resulting in a low Q value. While other conventional systems have attempted to reduce variance emanating from the coupling of electrodes to a substrate, reduction of diffractive losses remains unaddressed.
A need has, therefore, arisen for a surface acoustic wave system that curtails diffractive losses. A need has also arisen for such a surface acoustic wave system that provides for optimal Q value. A need has further arisen for such a surface acoustic wave system that has increased precision.
In the present invention, a surface acoustic wave system curtails diffractive losses and phase delay errors by shaping an acoustic wave for propagation. The surface acoustic wave system of the present invention provides for a high Q value. The surface acoustic wave system of the present invention has increased precision which improves its performance, particularly when used as a filter or oscillator.
In the surface acoustic wave system of the present invention, curved transmission elements are provided in order to shape a wave for propagation. Curved receiving elements are provided in order to match the shape and size of the propagating wave front, thereby fully receiving the wave at a definite arrival time and eliminating diffractive losses.
In one embodiment of the present invention, both the transmission and receiving elements are semi-circular in shape. The elements are curved concavely with respect to one another and satisfy a defined size and positional relationship. Alternatively, both transceiving elements of the present invention may be semi-elliptical or substantially parabolic in shape, providing necessary wave front matching.
In other embodiments, a transmission element may be shaped differently than, and curved either concavely or convexly with respect to, a receiving element; where the receiving element provides necessary wave front matching.
In one embodiment of the present invention, a transmission element may be linearly shaped and combined with a semi-circular receiving element to satisfy a defined size and positional relationship.
In yet another embodiment, a transmission element is curved either concavely or convexly with respect to, and combined with, an array of receiving elements, where each receiving element provides necessary wave front matching. This system provides a phase sensitive processing capability.